The present invention relates to a process for the production of acrolein or aqueous acrolein solution by dehydration of glycerol in the liquid or gaseous phase on acidic solid catalysts and to a method of using the aqueous acrolein solution in the production of 1,3-propanediol.
It is known that glycerol may be dehydrated in the presence of acidic substances. According to Organic Synthesis I, 15-18 (1964), acrolein is obtained at a yield of between 33 and 48% of theoretical by treating a mixture of powdered potassium hydrogen sulphate, potassium sulphate and glycerol at 190.degree. to 200.degree. C. The disadvantages of this process are not only the low yield but also the high weight ratio of dehydrating salts to glycerol which is required. This process is therefore unsuitable for the production of acrolein on an industrial scale.
The quantity of acidic catalyst used, such as sulfuric acid, may be kept low if dehydration of glycerol is performed in a homogeneous phase above the critical pressure of water (see S. Ramayya et al. in FUEL (Oct. 1987), Vol. 66, pages 1364-1371). Under the conditions stated in table 4 of that document (34.5 MPa, 350.degree. C., 0.5 molar aqueous glycerol solution), glycerol conversion is 39 to 55%, wherein the main products are acrolein and acetaldehyde in a ratio by weight of approximately 3:1 to 4:1. The considerable technical costs of working in the supercritical range, and the recycling or disposal of the sulfuric acid, make this process unattractive for industrial scale acrolein production.
The formation of acrolein from glycerol in the gaseous phase has also been investigated, for example under destructive gas chromatography conditions (Ishikawa Koichi et al., Bunseki Kagaku 32 (10) E 321-E 325, cited in Chemical Abstracts 101 (4): 32598w). A very dilute glycerol solution (1.5-150 mg/l) is pulsed over a destructive gas chromatography column coated with 10 to 30% KHSO.sub.4. The person skilled in the art receives no encouragement from this document to base an industrial acrolein production process on this analytical procedure because glycerol is only used at exceptionally low concentration and the column is practically unaffected by the pulsed reaction.
A process for the production of acrolein from glycerol is known from French patent FR 695,931, wherein glycerol vapors are passed over a fixed bed catalyst at over 300.degree. C. in particular at 400.degree. to 420.degree. C. The catalysts utilized are salts of tribasic acids or mixtures of such salts which may be on a support. According to the examples, pumice coated with 1% lithium phosphate or 1% iron phosphate is used. In this document, the acrolein yield of previously known liquid phase or gaseous phase processes using KHSO.sub.4 or MgSO.sub.4 is stated to be 20 or 30% respectively, and the yield of the claimed process according to the examples to be 75 or 80%.
The inventors of the present patent application duplicated the process of FR 695,931 and in so doing found that under the tested reaction conditions it was not possible to obtain the indicated yields with either lithium phosphate or iron phosphate. As is shown by the comparative examples, the acrolein yield was only approximately 1 to 3% at 300.degree. C. and 30 to 35% at 400.degree. C.; to a great extent, allyl alcohol, acetaldehyde and propionaldehyde are formed as secondary products. The disadvantage of the process according to FR 695,931 is thus the unsatisfactorily low selectivity of the reaction and consequently the low acrolein yield.
During investigation of model substances for biomass pyrolysis oils, the catalytic treatment of glycerol on H-ZSM5 zeolites at 350.degree. to 500.degree. C. was also investigated (see Dao, Le H. et al. ACS Symp. Ser., 376 (Pyrolysis Oils Biomass), 328-341 (1988), cited in Chem. Abstracts 110 (6): 41924n). Hydrocarbons are formed in only low yield, but reference is made to the formation of acrolein. As was found by the inventors of the present patent application, the dehydration selectivity of H-ZSM5 at, for example, 380.degree. C. in the liquid phase is hardly satisfactory. Furthermore, and this is of decisive importance for an industrial process, under the stated conditions the service life of the catalyst is limited to a few hours.